The present invention relates to a process and apparatus for the gas-phase polymerization of olefins, the polymerization being performed in a reactor having interconnected polymerization zones. In particular, the present invention is addressed to improve the operability of such a polymerization reactor by means of a novel design relative to the transport section connecting the interconnected polymerization zones of the reactor.
A novel gas-phase process for the olefin polymerization, which represents a gas-phase technology alternative to the fluidized bed reactor technology, is disclosed in the Applicant's earlier EP-B-782587 and EP-B-1012195. This polymerization process is carried out in a gas-phase reactor having two interconnected polymerization zones. The polymer particles flow upwards through a first polymerization zone (denominated as “the riser”) under fast fluidization or transport conditions, leave said riser and enter a second polymerization zone (denominated as “the downcomer”), through which they flow in a densified form under the action of gravity. A continuous circulation of polymer is established between the riser and the downcomer.
According to the description of EP-B-1012195 it is further possible to obtain, within this polymerization apparatus, two polymerization zones with a different monomers composition by feeding a gas/liquid stream (also denominated as “barrier stream”) to the upper part of the downcomer. Said gas/liquid stream acts as a barrier to the gas phase coming from the riser, and is capable to establish a net gas flow upward in the upper portion of the downcomer. The established flow of gas upward has the effect of preventing the gas mixture present in the riser from entering the downcomer. This polymerization process, described in detail in EP-B-1012195, reveals particularly useful when bimodal homopolymers or copolymers are aimed to be prepared.
The disclosure of the successive patent EP-B-1720913 further improves the flowability of the polymer particles flowing in a densified form along the downcomer. In particular, the claimed method involves the introduction of a liquid of condensed monomers, which is continuously fed along the height of the downcomer at a mass flow rate per unity of reactor surface higher than 30 Kg/(h m2). The percolation of this liquid onto the walls of the downcomer originates a liquid layer interposed between the polymer particles and the reactor wall, thus reducing the friction of the polymer onto the wall. As a result, the flowability of the polymer particles close to the downcomer walls is considerably improved. However, it has been discovered that the flow rate of descending liquid should not exceed certain values, otherwise the quick evaporation of relevant amounts of liquid in the downcomer may generate flows of vapor capable of locally fluidizing the polymer particles or locally generating a sluggish behavior of the descendent polymer. This clearly can disrupt the regular plug flow of the polymer along the downcomer, with the undesired effect of making non-homogeneous the residence time of the particles in the downcomer.
In order to overcome the above mentioned drawback, the description of WO2009/080660 tries to improve the operability of the downcomer highlighting the importance of a parameter, which is specific for a reactor having two interconnected polymerization zones. This parameter is the flow rate FP of polymer which is continuously transferred from the downcomer to the riser, thus establishing the continuous circulation of polymer between the two interconnected polymerization zones. The parameter FP may be also defined as the flow rate of polymer which by-passes the polymer discharge from the bottom part of the downcomer. According to the teaching of WO2009/080660 the amount of liquid barrier LB fed to the upper part of the downcomer should be strictly correlated with the flow rate FP of polymer continuously circulated between downcomer and riser. In particular, the ratio R between FP and LB should be maintained in a range comprised from 10 to 50, preferably from 15 to 45, in order to not disrupt the regular plug flow of the polymer descending along the downcomer, while at the same time ensuring a satisfying level of bimodality in the produced polyolefin.
However, another zone of high criticality in the correct working of the above polymerization reactor is represented by the transport section, which connects the bottom of the downcomer to the lower region of the riser and ensures the transfer of the polymer flow rate FP. Along this section the polymer particles coming from the downcomer has to be transferred at a high speed to the lower region of the riser: the severe conditions of high temperature, pressure and the high level of friction between polymer and wall may easily cause superficial melting of the polymer particles with the consequent generation of polymer chunks inside the transport section. The transfer of polymer between dowcomer and riser is generally achieved by means of pneumatic transport, i.e. by feeding a carrier gas to the inlet of the transport section. In order to achieve a more homogeneous distribution of the carrier gas within the transport section it may be useful to arrange a gas distribution grid. However, it has been observed that when such a distribution grid is arranged to cover only the inlet of such a carrier gas, a good flowability of the polymer along the transport section is not ensured. In fact, partial blocking in the polymer flow has been observed with consequent partial melting of the polymer and formation of agglomerates, which can also adhere onto the walls of the transport section. In the worst cases, the transport section may be even clogged by the presence of polymer agglomerates unable to reach the riser of the polymerization reactor.
It is therefore felt the need to improve the homogeneity of the polymer flow when passing from the downcomer to the riser along the transport section, so as to avoid obstructions and blocking in the flow of polymer circulated between the downcomer and the riser.